Add changelog for 0.6.0

[matthiasbeyer]

Preparation patches for the 0.6.0 release.

[matthiasbeyer]

@souze I had a closer look at the changelog and it seems to me that some stuff was also changed, so we cannot push out a patch-level release.

Instead, this prepares the 0.6.0 release for the crate.

What do you think?

[matthiasbeyer]

bors merge=petreeftime,matthiasbeyer

[bors[bot]]

Build failed:

• CI

[matthiasbeyer]

Meh. That's our sporadic fail. Let me

bors retry

this. :angry:

[bors[bot]]

Build failed:

• CI

[matthiasbeyer]

Rebased to latest master, but I think we should include the outstanding PRs as well...

[matthiasbeyer]

bors ping